prost = [2]
n=10000
for i in range(3,n+1):
    ksun=0
    for j in prost:
        if i%j==0:
            ksun=1
    if ksun==0:
        prost.append((i))

a = input()
b=[]
if in(a) in prost:
    b.append(a)
b.extend([a[:-1], a[1:], a[:-2], a[2:], a[1:-1], a[:-1], a[0], a[1], a[2], a[3]])

x=[]
for i in range(len(b)):
    s=b[i]
    for j in range(len(prost)):
        if int(s)==int(prost)):
            x.append(int(s))
if len(x)>0:
    print(max(x))
else:
    print(0)

